Sorry ,yes you apply it here ! 'C'est interdit' Not necessarily will be the consonant taken over as the initial sound of the following word, when it commences with a vowel as in 'ont'. In order for a liaison to be possible, it must end in a LATENT consonant. A latent consonant is simply a final consonant that is normally silent (as in petit ,ont ,interdit etc..). Example: Nous avons un petit enfant ( here the 's' in nous and 't' in petit are latent consonants.) C'est interdit ( here 't' is a latent consonant) Another example is 'et' ,the consonant 't' here is not a latent one, thus you don't liaise ' et aussi' ,you can google it ,it is not liased. Liaison operates in word sequences whose components are closely linked by sense, e.g : article + noun adjective +noun personal pronoun + verb etc.. That might help you know when to use it or not ,but usually with practice you will know automatically .

Hello Ludzik, You don't have to say "c'est_interdit". This liaison is not obligatory. Many French people just say "c'est interdit", just as they would say "c'est un chien" and "c'est au numéro 8" while others prefer to make the liaison: "c'est_interdit", "c'est_un chien", "c'est_au numéro 8". Yes, there's a rule. In fact, only a few liaisons should always be respected: please see http://en.wikipedia.org/wiki/Liaison_%28French%29#Obligatory_liaison